Interpandemic Period

Phase 1:  No new influenza virus subtypes have been detected in humans.  An influenza virus subtype that has caused human infection may be present in animals.  If present in animals, the risk of human infection or disease is considered low.

Phase 2:  No new influenza virus subtypes have been detected in humans.  However, a circulating animal influenza virus subtype poses a substantial risk of human disease.

Pandemic Alert Period

Phase 3:  Human infections(s) with a new subtype, but no human-to-human spread, or at most rare instances of spread to a close contact.

Phase 4:  Small cluster(s) with limited human-to-human transmission but spread is highly localized, suggesting that the virus is not well-adapted to humans.

Phase 5:  Large cluster(s) but human-to-human spread still localized, suggesting that the virus in becoming increasingly better adapted to humans, but may not yet be fully transmissible (substantial pandemic risk).

Pandemic Period

Phase 6:  Pandemic; increased and sustained transmission in general population.